Carbon fiber produced by vapor-phase growth method is collectively called as “vapor-grown carbon fiber” (hereinafter, sometimes referred to simply as “carbon fiber”), and the vapor-phase growth method, which is characterized in features such as enabling easy production of carbon fiber having a high aspect ratio, has been vigorously studied so far and therefore, there are a number of study reports made on the production method for vapor-grown carbon fiber. Carbon nanotube (i.e. carbon fiber whose diameter is in the nanometer order), which has been gathering attention recently, can be synthesized by an applied approach of vapor-phase growth method.
FIG. 1 is a cross-sectional view of a typical example of an apparatus for continuously producing carbon fiber by vapor-phase growth method. In FIG. 1, 1 is carrier gas, 2 is material hydrocarbon liquid, 3 is carrier gas flow rate regulator, 4 is carbon source compound gasifier, 5 is heater, 6 is supported catalyst, 7 is reaction furnace, 8 is carbon fiber collector and 9 is waste gas.
An example of the method generally employed is as follows. As raw material, hydrocarbon such as CO, methane, acetylene, ethylene, benzene or toluene is used. If the hydrocarbon material 2 is gas at room temperature, the gaseous material is supplied in gaseous state into the reactor after mixed with carrier gas 1, on the other hand, if the hydrocarbon material 2 is liquid at room temperature, the material is supplied into the reactor either after gasified by gasifier 4 and mixed with carrier gas (an example shown by FIG. 1) or the liquid material is sprayed into the heating zone of the reactor. As carrier gas, nitrogen gas which is an inert gas, hydrogen gas which is reducing gas or the like is employed. As catalyst, a supported catalyst 6 where a metal is supported on a support such as alumina or an organometal compound such as ferrocene is used. In case of using a supported catalyst 6, after subjecting the supported catalyst to necessary pretreatment such as placing the supported catalyst in the reaction zone and heating the catalyst, the hydrocarbon material 2 is supplied into the reactor to react (an example shown by FIG. 1). Alternatively, the catalyst subjected to pretreatment is supplied continuously or pulse-wise from outside the reaction zone to allow the reaction. Further alternatively, an organometal compound such as ferrocene which is a uniform-type catalyst precursor is fed together with the hydrocarbon material into the heating zone continuously or pulse-wise, to thereby produce carbon fiber by using as catalyst, metal particles generated in thermal decomposition of the catalyst precursor. The product generated by the reaction is gathered in the inside of the heating zone or in collector 8 present at the end of the heating zone, and after reaction of a predetermined time, the product is collected.
Methods for producing carbon fiber by vapor-phase growth are roughly classified by process of feeding catalyst or catalyst precursor of the catalyst into the following groups.
(a) method where a substrate or board of alumina or graphite which supports a catalyst or a catalyst precursor is placed in the heating zone, and then the catalyst is contacted with hydrocarbon supplied in gas form;
(b) method where particles of a catalyst or a catalyst precursor is dispersed in liquid hydrocarbon, and supplied continuously or pulse-wise into the heating zone from outside the reaction zone to contact the catalyst with the gasified hydrocarbon at a high temperature;
(c) method where metallocene or a carbonyl compound which is soluble in liquid hydrocarbon is used as a catalyst precursor, and liquid hydrocarbon having the catalyst precursor dissolved therein is supplied into the heating zone, to thereby contact the catalyst with the hydrocarbon at a high temperature; and
(d) method where a gas obtained by evaporation by heating a catalyst precursor having a relatively high vapor pressure or by sublimating such a compound is contacted with hydrocarbon gas in the heating zone.
Method (a) requires procedures to be conducted each independently, i.e., spreading a catalyst or its precursor on a substrate, when necessary subjecting the catalyst or its precursor to pretreatment and collecting carbon fiber thereby produced after the temperature lowered and therefore, the production process cannot proceed continuously, which leads to low productivity. Also, the method, involving many steps of preparation of catalyst, coating a substrate with the catalyst, pretreatment of reducing to a metallic state, generation of carbon fiber and collecting produced carbon fiber from the substrate, is economically disadvantageous.
On the other hand, methods (b), (c) and (d) where production process can proceed continuously are more excellent in productivity than method (a).
However, in method (b), in supplying particles of a catalyst or catalyst precursor which have been dispersed in liquid hydrocarbon from outside the reaction zone into the heating zone continuously or pulse-wise, uniform suspension cannot be obtained, which leads to a problem that the supply ratio of hydrocarbon/catalyst cannot be stabilized. In order to solve the problem, a method where suspension with surfactant added thereto is supplied (Patent Document 1) and a synthesis method of single-layer carbon nanotube wherein suspension prepared by suspending in hydrocarbon such as toluene, catalyst particles having a uniform diameter in the nanometer order, i.e. a microemulsion is continuously supplied into the heating zone (Patent Document 2), have been reported. However, they are not necessarily satisfactory methods, and these methods, which require a step of preparing a suspension, is economically disadvantageous.
Meanwhile, method (c) uses as catalyst precursor, metallocene or a carbonyl compound which is soluble in liquid hydrocarbon. In this case, since combination of the hydrocarbon and the catalyst precursor is limited by the solubility, there remains a problem that a necessary amount of the catalyst precursor cannot be dissolved in the hydrocarbon so that the resulting amount of the catalyst is insufficient.
Further, in methods (b) and (c), hydrocarbons usable therein include only those which are liquid at room temperature.
Method (d) where a catalyst precursor is gasified in advance and independently supplied is advantageous in that optimum selection of catalyst source and its concentration can be made freely, unlike method (b) requiring a step of improving dispersibility in hydrocarbon and unlike method (c) where catalyst is limited by solubility in hydrocarbon.